Multiple hypotheses explain the possible connections between sleep and learning in humans. Research indicates that sleep does more than allow the brain to rest. It may also aid the consolidation of long-term memories.
REM sleep and slow-wave sleep play different roles in memory consolidation. REM is associated with the consolidation of nondeclarative (implicit) memories. An example of a nondeclarative memory would be a task that we can do without consciously thinking about it, such as riding a bike. Slow-wave, or non-REM (NREM) sleep, is associated with the consolidation of declarative (explicit) memories. These are facts that need to be consciously remembered, such as dates for a history class.
REM sleep and slow-wave sleep play different roles in memory consolidation. REM is associated with the consolidation of nondeclarative (implicit) memories. An example of a nondeclarative memory would be a task that we can do without consciously thinking about it, such as riding a bike. Slow-wave, or non-REM (NREM) sleep, is associated with the consolidation of declarative (explicit) memories. These are facts that need to be consciously remembered, such as dates for a history class.
Increased learning
Popular
sayings can reflect the notion that remolded memories produce new
creative associations in the morning, and that performance often
improves after a time-interval that includes sleep. Current studies demonstrate that a healthy sleep produces a significant learning-dependent performance boost.
The idea is that sleep helps the brain to edit its memory, looking for
important patterns and extracting overarching rules which could be
described as 'the gist', and integrating this with existing memory.
The 'synaptic scaling' hypothesis suggests that sleep plays an
important role in regulating learning that has taken place while awake,
enabling more efficient and effective storage in the brain, making
better use of space and energy.
Healthy sleep must include the appropriate sequence and proportion of NREM and REM phases,
which play different roles in the memory consolidation-optimization
process. During a normal night of sleep, a person will alternate between
periods of NREM and REM sleep. Each cycle is approximately 90 minutes
long, containing a 20-30 minute bout of REM sleep.
NREM sleep consists of sleep stages 1–4, and is where movement can be
observed. A person can still move their body when they are in NREM
sleep. If someone sleeping turns, tosses, or rolls over, this indicates
that they are in NREM sleep. REM sleep is characterized by the lack of
muscle activity. Physiological studies have shown that aside from the
occasional twitch, a person actually becomes paralyzed during REM sleep. In motor skill
learning, an interval of sleep may be critical for the expression of
performance gains; without sleep these gains will be delayed.
Procedural memories are a form of nondeclarative memory, so they would most benefit from the fast-wave REM sleep. In a study, procedural memories have been shown to benefit from sleep.
Subjects were tested using a tapping task, where they used their
fingers to tap a specific sequence of numbers on a keyboard, and their
performances were measured by accuracy and speed. This finger-tapping
task was used to simulate learning a motor skill. The first group was
tested, retested 12 hours later while awake, and finally tested another
12 hours later with sleep in between. The other group was tested,
retested 12 hours later with sleep in between, and then retested 12
hours later while awake. The results showed that in both groups, there
was only a slight improvement after a 12-hour wake session, but a
significant increase in performance after each group slept. This study
gives evidence that REM sleep is a significant factor in consolidating
motor skill procedural memories, therefore sleep deprivation can impair
performance on a motor learning task. This memory decrement results
specifically from the loss of stage 2, REM sleep.
Declarative memory
has also been shown to benefit from sleep, but not in the same way as
procedural memory. Declarative memories benefit from the slow-waves nREM
sleep. A study
was conducted where the subjects learned word pairs, and the results
showed that sleep not only prevents the decay of memory, but also
actively fixates declarative memories.
Two of the groups learned word pairs, then either slept or stayed
awake, and were tested again. The other two groups did the same thing,
except they also learned interference pairs right before being retested
to try to disrupt the previously learned word pairs. The results showed
that sleep was of some help in retaining the word pair associations, while against the interference pair, sleep helped significantly.
After sleep, there is increased insight. This is because sleep
helps people to reanalyze their memories. The same patterns of brain
activity that occur during learning have been found to occur again
during sleep, only faster. One way that sleep strengthens memories is by
weeding out the less successful connections between neurons in the
brain. This weeding out is essential to prevent overactivity. The
brain compensates for strengthening some synapses (connections) between
neurons, by weakening others. The weakening process occurs mostly
during sleep. This weakening during sleep allows for strengthening of
other connections while we are awake. Learning is the process of
strengthening connections, therefore this process could be a major
explanation for the benefits that sleep has on memory.
Research has shown that taking an afternoon nap increases learning capacity. A study
tested two groups of subjects on a nondeclarative memory task. One
group engaged in REM sleep, and one group did not (meaning that they
engaged in NREM sleep). The investigators found that the subjects who
engaged only in NREM sleep did not show much improvement. The subjects
who engaged in REM sleep performed significantly better, indicating that
REM sleep facilitated the consolidation of nondeclarative memories. A more recent study
demonstrated that a procedural task was learned and retained better if
it was encountered immediately before going to sleep, while a
declarative task was learned better in the afternoon.
Electrophysiological evidence in rats
A 2009 study
based on electrophysiological recordings of large ensembles of isolated
cells in the prefrontal cortex of rats revealed that cell assemblies
that formed upon learning were more preferentially active during
subsequent sleep episodes. More specifically, those replay events were
more prominent during slow wave sleep
and were concomitant with hippocampal reactivation events. This study
has shown that neuronal patterns in large brain networks are tagged
during learning so that they are replayed, and supposedly consolidated,
during subsequent sleep. There have been other studies that have shown
similar reactivation of learning pattern during motor skill and
neuroprosthetic learning. Notably, new evidence is showing that reactivation and rescaling may be co-occurring during sleep.
Sleep in relation to school
Sleep
has been directly linked to the grades of students. One in four U.S.
high school students admit to falling asleep in class at least once a
week. Consequently, results have shown that those who sleep less do poorly. In the United States,
sleep deprivation is common with students because almost all schools
begin early in the morning and many of these students either choose to
stay awake late into the night or cannot do otherwise due to delayed sleep phase syndrome. As a result, students that should be getting between 8.5 and 9.25 hours of sleep are getting only 7 hours. Perhaps because of this sleep deprivation, their grades are lower and their concentration is impaired.
As a result of studies showing the effects of sleep deprivation
on grades, and the different sleep patterns for teenagers, a school in New Zealand,
changed its start time to 10:30 a.m., in 2006, to allow students to
keep to a schedule that allowed more sleep. In 2009, Monkseaton High
School, in North Tyneside,
had 800 pupils aged 13–19 starting lessons at 10 a.m. instead of the
normal 9 a.m. and reported that general absence dropped by 8% and
persistent absenteeism by 27%. Similarly, a high school in Copenhagen has committed to providing at least one class per year for students who will start at 10 a.m. or later.
College students represent one of the most sleep-deprived
segments of our population. Only 11% of American college students sleep
well, and 40% of students feel well rested only two days per week.
About 73% have experienced at least some occasional sleep issues. This
poor sleep is thought to have a severe impact on their ability to learn
and remember information because the brain is being deprived of time
that it needs to consolidate information which is essential to the
learning process.
